1. Field of the Invention
The present invention relates to an optical recording/reproducing apparatus such as an optical disk apparatus. More particularly, the invention relates to an optical recording/reproducing apparatus with a plurality of laser beam sources which can perform overwrite and reproduction for verification immediately after recording, substantially at the same time and in parallel. The optical recording/reproducing apparatus of the present invention is effectively applicable to magneto-optical recording/reproducing apparatus.
Also, the present invention relates to a semiconductor laser array which has a plurality of laser units on a single substrate and which can drive them independently of each other.
2. Related Background Art
Recent research and development has been vigorous for increasing the transfer rate of magneto-optical disk apparatus. Presently commercially available magneto-optical disk apparatus require three rotations of a disk for erasure, for recording and for reproduction (verification) in data writing, and have a drawback that the data transfer rate, especially in recording, is lower than that in use of a hard disk or the like. Then, there are various apparatus proposed, for example a recording/reproducing apparatus and a recording medium of overwrite type in which recording and erasure are carried out during a rotation of a disk, a recording/reproducing apparatus in which a plurality of light spots are formed to carry out verification right after recording, and an apparatus in which a plurality of light spots are formed to carry out recording and reproduction in parallel.
Proposed as an overwritable magneto-optical disk apparatus is one of a type in which a magnetic field applied to a magneto-optical recording medium is modulated according to record information, as described in Japanese Laid-open Patent Application No. 51-107121. In addition to that, another magneto-optical disk apparatus is proposed for example in Japanese Laid-open Patent Application No. 64-82348, in which a series of erasure-record-reproduction processes are finished during a rotation of a disk with beams for recording and for verification being located on a track. These are categorized as a magnetic field modulation overwrite method, because the magnetic field applied to the magneto-optical record medium is modulated according to record information. Also, there are record media which are overwritable by modulating a light beam for writing information into magneto-optical recording media, as proposed for example in Japanese Laid-open Patent Application No. 62-175948 or in Japanese Laid-open Patent Application No. 63-268103. These record media enable overwrite by the structure that multiple magnetic layers different in properties of Curie temperature and coercivity are exchange-coupled with each other. These are categorized as an optical modulation overwrite method.
Further, for example Japanese Laid-open Patent Application No. 54-146613 or Japanese Laid-open patent Application No. 64-19535 describes apparatus in which a plurality of light sources form a plurality of light spots on adjacent tracks on a recording medium to perform parallel recording/reproduction. These achieve parallel recording and reproduction by using a semiconductor laser array as a light source and an optical system almost identical to that in conventional magneto-optical disk apparatus using a single light source.
As described above, the magneto-optical disk apparatus can be improved to increase the data transfer rate up to that of a hard disk or more than that, utilizing the property that the optical system allows multiplex transmission of plural light beams.
If an attempt is made to execute overwrite and verification immediately after recording by a plurality of laser beam sources within a rotation of a disk and in parallel in order to further increase the transfer rate of the magneto-optical disk apparatus, combinations of conventional technology make the construction of an optical system too complex to achieve the attempt.
For electrophotographic printers or optical information recording/reproducing apparatus using a semiconductor laser as a light source, a method is being studied for simultaneous and parallel processing of a plurality of lines or a plurality of tracks with a plurality of laser beams and via a single optical system in order to improve the processing speed.
FIG. 1 is a schematic drawing to show an optical system in a conventional optical information recording/reproducing apparatus. In FIG. 1, reference numeral 112 denotes a light source, 113 a collimator lens, 114 a beam splitter, 115 an objective lens, 116 a record medium, 117 and 120 condenser lenses, 118 a light quantity detecting sensor, 119 a half-wave plate, 121 a cylindrical lens, 122 a polarization beam splitter, 123 a first signal detecting sensor, and 124 a second signal detecting sensor. A plurality of beams from the light source 12 are aligned in the direction of arrows 125 in FIG. 1. Each beam emitted from the light source 112 is split into two by the beam splitter 114, reflected light thereby passes through the condenser lens 117 to be focused on the light quantity detecting sensor 118. A light quantity of each beam is detected by a sectioned light receiving portion (not shown) for each beam. Transmitted light by the beam splitter 114 is converged by the objective lens 115 to form a spot on the record medium 116, with which information recording or reproduction is carried out. Reflected light by the record medium 116 is again reflected by the beam splitter 114, and a plane of polarization of the reflected light is rotated by 45 degrees by the half-wave plate 119. Then, the light is guided to pass through the condenser lens 120 and the cylindrical lens 121 to form astigmatic beams. The polarization beam splitter 122 further guides the beams to the first and second signal detecting sensors 123, 124. A light receiving surface (not shown) of a first or second signal detecting sensor is so divided as to independently receive the plural beams of reflected light corresponding to the beams from the light source 112. In particular, one of the sensor units in the first signal detecting sensor is formed as a quarter-sectioned sensor, which detects a track deviation and a defocus amount of a beam converged from an astigmatic beam onto the record medium. The first and second signal detecting sensors reproduce information recorded in the record medium by detecting a differential output between two separate light receiving portions, which are for detecting respective beams from a same beam. A permanent magnet 90 applies an external magnetic field as an auxiliary magnetic field to a micro region of a beam spot in an information record.
FIG. 2 shows a positional relation between the beams and the tracks on the record medium. Each of the beams from the light source 112 is positioned on a track of the record medium, and the lasers emitting the beams are operated independent of each other, which enables parallel recording and reproduction of up to the number of beams.
Since return light from the record medium is coupled with the original beam in reproduction and thereby causes return light induced noises, a measure is normally employed for reducing the coherence of the laser by providing an external high-frequency overlay circuit.
It is important that the light source employed in such an apparatus shown in FIG. 1 have a high output power and even properties of laser beams, in order that each laser for emitting a beam is effective for recording and reproduction. A conventional light source used for such a purpose is a semiconductor laser array integrated on a single substrate in the monolithic manner. FIG. 3 shows the structure of a conventional semiconductor laser array. This is an example of a semiconductor laser array of an inner strip type produced by the liquid phase epitaxy (LPE).
In FIG. 3, reference number 101 designates a p-type GaAs substrate for crystal growth thereon, 102 an n-type GaAs current block layer for current constriction in the inner stripe structure, 103, 104, 105 a p-type AlGaAs first clad layer, an AlGaAs active layer, and an n-type AlGaAs second clad layer, respectively, constituting a double heterojunction, 106 an n-type GaAs cap layer, 107, 108 electrodes, 110 an integrated laser emission portion, and 111 a separation groove for enabling independent drive of the integrated lasers.
The production process of the semiconductor laser array is next described.
First crystal growth is carried out to form a block layer 102 on a flat substrate 101. Etching is next done to form grooves for inner stripes, whereby the block layer 102 has stripes in width (W2) at intervals of about 100 .mu.m and grooves in depth reaching the substrate 101. Next, second crystal growth is carried out to form layers 103-106. The first clad layer 103 is formed while preferentially filling up the etched grooves, which is the property of liquid phase epitaxy, and has a substantially flat surface above the grooves. Accordingly, the active layer 104, the second clad layer 105 and the cap layer 106 above the third layer are also formed in a substantially parallel layer to the substrate 101 by crystal growth. The first and second clad layers and the active layer 104 therebetween form a double heterojunction to be a laser waveguide. Then, a separation groove 111 is formed to electrically separate the integrated laser waveguides from each other by etching the middle portion between radiative portions 110 from the top of cap layer 106 to the substrate 101. A cathode electrode 108 is formed on each of the n-type cap layers 106 separated by the separation groove 111, while an anode electrode 107 is formed on the bottom of the p-type substrate 101.
The conduction type of each layer may be reversed in the above semiconductor layer array.
When a voltage is applied between the electrodes, a current flowing between the electrodes is constricted by the block layer 102. The constricted current is efficiently injected into the limited active layer region on the groove to oscillate the laser. A radiative portion 110 is in the active layer 104 while being located above the etched groove in the block layer 102. Since the cathode electrodes 108 on the cap layers 106 are separated from each other by the separation groove 111, an arbitrary laser may be independently driven by turning on and off a corresponding cathode electrode. The far field pattern FWHM (full-width at half maximum) beam divergence angle in the direction parallel to the junction plane (.THETA..parallel.) is generally determined by the stripe width while the far field pattern FWHM beam divergence angle in the direction normal to the junction plane (.THETA..perp.) is generally determined by the thickness of the active layer. .THETA..parallel. is designed to be about 9 degrees in order to obtain a high output property over 30 mW necessary for recording and to decrease the astigmatic difference in low output. Also, .THETA..perp. is often designed to be below about 25 degrees to obtain the high output property. As described above, the conventional semiconductor laser array can provide beams with high output and equal property, and therefore is used as a light source effective for parallel processing in electrophotographic printers or in optical information recording/reproducing apparatus.
The conventional semiconductor laser array, however, has such an arrangement that high output lasers having the same property of a rated output of about 30 mW are integrated therein, and is evaluated as to the low output property in reproduction by the characteristics shown upon operation of the high output lasers at about 3 mW. A high output laser needs to have an increased stripe width in order to assure the high output property necessary for recording, which causes a problem of increase in astigmatic difference upon low output operation. Also, since the wavelength of the laser becomes longer in proportion to the operation current, there is a wavelength difference between a recording beam and a reproducing beam of about 3-5 nm. If the beams are focused through the same optical system onto a track, the variation of astigmatic difference or the wavelength difference will cause relative defocus or degradation of recording or reproduction quality.
In addition, since the noise property is considerably degraded by the coupling of the laser beam with return light in reproduction, the external high-frequency modulation is necessary, which requires an extra circuit.
It is an object of the present invention to provide a semiconductor laser array most suitable to increase the transfer rate of an optical information recording/reproducing apparatus. Specifically, two beams are positioned on a same track, a forward beam of which is used for recording and a following beam of which is for reproduction. This arrangement enables the verification operation immediately after recording, which omits the rotation wait time so as to increase the transfer rate.
FIG. 4 shows the construction of another conventional semiconductor laser array which can be used in the apparatus shown in FIG. 1. In FIG. 4, the same portions are denoted by the same reference numerals as those in FIG. 3. The semiconductor laser array of FIG. 4 is different from that of FIG. 3 in that the array has four laser units, and may be produced in the same manner.
FIG. 5 shows the positional relation between tracks and beam spots on a record medium in a case of this semiconductor laser array being used. If a beam spacing is D between beam spots from the light source 112 and if the track pitch of the record medium is L, a laser beam can be positioned on each track when a line of the spots is rotated by .THETA.=sin.sup.-1 (L/D) with respect to the track direction. The laser units emitting respective beams can be controlled independent of each other, so that recording or reproduction can be carried out in parallel in the number of beams.
Since all radiative points are arranged at the same height from the same substrate in the above conventional example, they are aligned on a straight line on the record medium, which is convenient for parallel processing of all radiative points. This arrangement, however, never permits at least two radiative points to be located on a same track and other radiative points to be located on a track adjacent to the track. The optical information recording/reproducing apparatus needs erasure, recording and reproduction processes for a recording operation, so that three rotations of a disk are necessary at maximum for completing the recording operation. A method for completing the erasure, recording and reproduction processes within a rotation of a disk is to perform consecutive processes thereof with two spots being positioned on a same track.